Jim: A '65 GT350 would have had mostly black, some may see it as dark gray rope caulk used in most places where called out. Although, I have found gray caulk used in places like under the sill plates.
The old 3M product was correct, but over the years, they changed the formula to the butyl-based style, which was never used on these cars. There are other correct options as you've found.
Hope you never have to remove the gas tank, it will be a sticky mess!